I've tried mashing in varying amounts of canned or baked pumpkin to raise the gravity above that predicted from the grain, and to my surprise the starchy gourd didn't appear to have much of an effect. Then again my efficiency is all over the place with my Corona mill, so it's hard to tell. I wanted to see what effect the pumpkin had without spices, and my conclusion was not much. I make a few with baked pumpkin, and the more I used the cloudier it got. Canned pumpkin came out clear if I remember correctly. That's not to say that it didn't add to the flavor. It did. But not as much as the spices.
I wonder what would happen if you made two otherwise identical batches of spiced ale, one with pumpkin and one without. I just might try it. Then again I probably won't. I've no use for ten gallons of spiced ale. One batch is enough.
That baked pumpkin ended up in the freezer in 1# bags, and many of those bags ended up in brews. Just adding a pound into the mash for the hey of it. Some came out quite yummy, though I doubt the pumpkin had much to do with it. One was super good. It was a dark ale. 8.5# pale malt, 8oz crystal, 1oz each black patent, chocolate malt and roasted barley, and of course one pound of baked pumpkin, all mashed at 150. Tettenanger hops 1oz for 60, .25 oz 10 min and dry hop. Safeale US-05. SG 1.046 F.G 1.007. I bet that one would taste pretty good with a bit of clove and nutmeg, and maybe a bit more pumpkin. Canned has more flavor.